The present application claims priority to Japanese Application No. P2000-010206 filed Jan. 14, 2000, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to a positive electrode material capable of occluding and releasing a light metal and a secondary battery using the positive electrode material.
2. Description of the Related Art
In recent years, with progress in electronic techniques, a small portable electronic device typified by a camcorder (video cassette recorder), a portable telephone, or a laptop computer is being developed. Consequently, as a driving source of the device, attention is paid to a small and light lithium ion secondary battery capable of obtaining high energy density.
As a positive electrode material of the lithium ion secondary battery, for example, a lithium composite oxide such as lithium cobalt composite oxide, a lithium nickel composite oxide, or a lithium manganese composite oxide is very promising since it realizes a large capacity.
When such a lithium composite oxide is used as a positive electrode material, however, the lithium composite oxide and an electrolyte react with each other as charging and discharging are repeated, a reaction product is deposited on the surface of the positive electrode. It promotes polarization and causes a problem such that a charging/discharging cycle characteristic, shelf stability, and a discharge load characteristic deteriorate.
Japanese Unexamined Patent Application No. HEI 9-147836 discloses a lithium secondary battery in which a predetermined metal element is carried on the surface of the positive electrode. In the case where a metal is carried, it is feared that the carried metal elutes and exerts an influence on a battery reaction. Japanese Unexamined Patent Application No. HEI 8-102332 discloses a secondary battery in which a low-active oxide or a low-active metal having a property of electrochemically decomposing a nonaqueous electrolytic solution rather than a lithium oxide is dispersed and held in a part of the particulate surface of a positive electrode active substance. When the carried metal is dispersed and held, however, there is the possibility of the elution. When a low-active oxide such as CaO is dispersed and held, a problem such that conductivity in the positive electrode cannot be sufficiently obtained arises.
The invention has been achieved in consideration of the problems and its object is to provide a positive electrode material having improved charging/discharging cycle characteristic, shelf stability, and discharge load characteristic by preventing a reaction product from being deposited on the surface of the positive electrode by suppressing elusion of the positive electrode material while assuring conductivity, and to provide a secondary battery using the positive electrode material.
In a positive electrode material according to the present invention, a coating portion made of a conductive oxide is provided on at least a part of a particulate substance capable of occluding and releasing a light metal, and the quantity of the conductive oxide coated is in a range from 0.001 mol to 0.1 mol per 1 mol of the particulate substance.
A secondary battery according to the present invention has a positive electrode, a negative electrode, and an electrolyte, the positive electrode includes a positive electrode material in which a coating portion made of a conductive oxide is provided on at least a part of a particulate center portion capable of occluding and releasing a light metal, and a quantity of the conductive oxide coated in the positive electrode material is in a range from 0.001 mol to 0.1 mol per 1 mol of the particulate substance.
In the positive electrode material according to the present invention, since the coating portion made of a conductive oxide is provided on at least a part of the center portion, elution is suppressed and high conductivity is realized.
In the secondary battery according to the invention, since the positive electrode uses the positive electrode material of the invention, the conductivity in the positive electrode is assured, elution of the positive electrode material is suppressed, and a reaction product caused by the elution of the positive electrode material can be prevented from being made and deposited on the surface of the positive electrode.
Other and further objects, features and advantages of the invention will appear more fully from the following description.